1. Field of the Invention
This invention relates to a colour display device.
2. Discussion of Prior Art
Colour display devices such as colour television and colour projection apparatus are well known. One variety of colour television involves a thermionic vacuum tube containing a phosphor pixel screen. The screen has three phosphor dots in respective primary colours at each pixel. Three electron guns are employed, each addressing a respective primary colour dot at each pixel. This arrangement is complex, bulky and expensive. Colour projection apparatus typically employs a collared slide projected on to a screen with illumination by a white light source. This does not land itself to the projection of television signals, since such slides are only suitable for still images.
Liquid crystal displays are known for the production of television pictures, but there is difficulty in producing sufficiently large displays of the required quality. To obtain a large area picture, it is known to employ three liquid crystal displays each illuminated by a respective primary colour. Light from each display is projected on to a screen at which the primary colours are superimposed to yield a colour image. This is optically complex, as it requires three optical sections with accurate mutual alignment.
Colour display systems incorporating liquid crystal light valves or spatial light modulators are also known. International Patent Application No. PCT/US88/00142 published as No. WO88/06391 relates to one such system. It includes a white light source and an array of dichroic mirrors to define separate red, green and blue light beams. The beams pass through respective polarizers referred to as optical switches; the latter are arranged to provide either polarization rotation or no such rotation according to an electrical signal input effecting an "off" or "on" state. The beams are subsequently recombined to form a single beam and pass to a polarization analyser. The analyser reflects light with rotated polarization to a light valve and transmits light having unchanged polarization. The red, green and blue beams undergo rotation of polarization successively, so that at any one time the light valve receives light of one of the three colours. A cathode ray tube illuminates the light valve to form an image therein, and the image reflects the incident red, green or blue light to a projection screen or the like. This system is undesirably complex. It requires five polarization selective devices and three colour selective (dichroic) mirrors.
European Patent Application No. 0301 715 A1 relates to a display system in which a display matrix is illuminated by beams of different collared light from a light source. The light source may comprise a single filtered white light source, or a plurality of collared sources. Liquid crystal shutters are arranged adjacent the light source and are switchable between a light blocking mode and a light transmitting mode to allow a single collared light beam to illuminate the display matrix. As the liquid crystal shutters are adjacent to the light source they may be affected by heating up the source. Additionally, the embodiment employing the white light source has an efficiency of only 1/9 or 11%. This is because only one-third of the light incident on the shutters is utilised and only one-third of the light utilised is allowed through the shutters.
The published International Application Number WO 90/05429 also relates to a display system in which a display is illuminated by a white light source via liquid crystal shutters with red, green and blue filters to provide a single collared illumination beam. This device also has a display efficiency of only 11%.